The present invention relates to a defect measuring method and apparatus for an object to be measured. Particularly, the present invention is concerned with a defect measuring method suitable as a method for measuring and evaluating defects in crystal present in a surface layer of a semiconductor wafer, as well as an apparatus using the method.
Heretofore, a method and an apparatus for non-destructively measuring defects in crystal present in a surface layer of a semiconductor wafer have been publicly known.
For example, for the observation of defects present in a surface layer portion not deeper than 5 xcexcm, there has been made available a visible laser scattering tomography, for example, such as MO521 of Mitsui-Kinzoku-Kozan. According to this tomography, it is possible to observe defects up to a depth of 5 xcexcm on an average from silicon wafers, but the depth and size of each individual surface layer defect cannot be obtained.
Also, in Japanese Patent Laid Open No. 64136/97 there are disclosed a method and an apparatus for determining an intensity distribution of scattered beams which reflect actual sizes of defects present in a surface layer of a semiconductor wafer. According to this method, defects present in a semiconductor wafer are measured on the basis of scattered beams generated from these defects. More particularly, light having a silicon absorbing wavelength is radiated to the wafer from a light source through an optical system, allowing scattered light to be generated from the wafer. This scattered light,is received by a detector and the optical system. Such an operation is performed for the whole area of the wafer and the results of scanning are processed as defect indicating information by means of a computer. At this time there is made correction for eliminating the influence of light absorption by silicon. The results of measurement are corrected taking the attenuation rate of light into account.
There also has been proposed an apparatus wherein plural laser beams of different wavelengths are radiated to a wafer, and on the basis of a difference in absorbance between the wavelengths there is obtained a scattered light intensity which reflects the size of a defect or information relating to the depth of a defect from a polished specular surface.
If the conventional laser scattering tomography of a two-wavelength type is used for the same purpose, the depth of a defect up to 0.5 xcexcm right beneath the surface of silicon wafer can be measured by utilizing the wavelength dependence of absorption coefficient. For example, in OSDA (optical shallow crystal defect analyzer) of Hitachi there are used two laser beams of 532 nm and 810 nm in wavelength.
According to the conventional infrared laser scattering tomography, it is possible to measure defects in silicon wafers present at a position deeper than about 10 xcexcm from the surface, but the measurement at the shallower region is impossible. An example of laser beam wavelength is 1.06 xcexcm.
The semiconductor wafer defect measuring method and apparatus disclosed in Japanese Patent Laid Open No. 64136/97 are for making correction with respect to a scattered light intensity distribution obtained by measurement and not for correcting the intensity of a scattered light with respect to each detect. Thus, the depth from a polished specular surface, as well as a relative dimensional factor, of each crystal defect cannot be measured.
On the other hand, in a measuring apparatus using plural laser beams of different wavelengths, the optical system used is complicated, which is very disadvantageous in point of detection accuracy and cost.
The above conventional apparatuses are lacking in a method for obtaining, with a high accuracy, information relating to the depths and sizes of defects present in a surface layer up to a depth of 10 xcexcm from the surface. For example, it has been difficult to determine which of two defects is the larger.
In the case where a high accuracy is not so strictly required for the depth of a subsurface defect and it suffices roughly to know a defect density up to a certain depth, the measurement concerned can be effected by changing the laser wavelength. In this case, however, it is necessary to provide plural lasers in advance. Thus, this method is also compelled to use a complicated optical system, which is disadvantageous in point of detection sensitivity and cost.
It is an object of the present invention to provide a method and apparatus for measuring defects of an object (especially a semiconductor wafer) to be measured which method and apparatus can measure the depth of each individual defect and the number of defects in a non-destructive manner.
It is another object of the present invention to provide a semiconductor wafer defect measuring method and apparatus capable of comparing sizes of defects.
It is a further object of the present invention to provide a semiconductor wafer crystal defect measuring method and apparatus using an optical system not complicated, advantageous in point of detection sensitivity and cost, and capable of measuring defects in a non-destructive manner.
According to the present invention, crystal defect measuring method and apparatus for an object (especially a semiconductor wafer) to be measured are described in the appended claims. The defects include not only oxygen precipitates, but also voide defects, stacking faults and others that produce scattered beams upon receipt of laser beams.
The present invention is to improve a method and apparatus for non-destructively measuring defects present in a subsurface layer of an object to be measured.
According to the present invention, the phenomenon that the intensity of a scattered light varies depending on temperature is utilized, and the intensities of scattered beams are measured at two or more temperatures, thereby determining the depths of defects from the surface of an object (especially a semiconductor wafer) to be measured, as well as relative dimensional factors of the defects.
In the present invention, the phenomenon that the penetration depth of a laser beam varies depending on temperature is utilized, and the intensity of a scattered beam is measured at any of plural temperatures, thereby determining the number of crystal defects present in a region from the wafer surface up to an arbitrary depth.
In a preferred example of the defect measuring method and apparatus according to the present invention, the intensities of scattered beams of a laser at two or more temperatures of an object to be measured are measured, thereby determining the depths of defects from the surface of the object to be measured. In comparing the sizes of defects, the intensities of scattered beams of a laser at two or more temperatures of an object to be measured are measured to determine a relative dimensional factor of each defect in the object. In determining the number of defects, a laser beam is radiated to the surface of an object to be measured to scan the same surface and the intensity of scattered beam from the object is measured at any of plural temperatures of the object, thereby calculating the number of defects present in a region from the object surface up to an arbitrary depth. A typical and preferred example of the object to be measured is a semiconductor wafer, particularly a silicon wafer, provided the present invention is also applicable to other objects than wafer.
In one preferred mode for carrying out the present invention, the intensities of scattered beams of a single laser are measured at two or more temperatures, thereby measuring defects present in a surface layer of a semiconductor wafer in a non-destructive manner. For example, by measuring the intensities of scattered beams generated from defects in a semiconductor wafer at two different temperatures, there are determined the depths of defects from the surface of the wafer, as well as relative dimensional factors of these defects.
In another mode for carrying out the present invention, the intensity of a scattered beam of a single laser is measured at any of plural temperatures, thereby measuring a defect present in a surface layer of a semiconductor wafer in a non-destructive manner. For example, by measuring the intensity of a scattered beam at any of plural temperatures of a wafer, there is calculated the number of crystal defects present in a region from the wafer subsurface up to an arbitrary depth.
In a further mode for carrying out the present invention, the number of defects present in a first wafer subsurface layer and that present in a second wafer subsurface layer are measured at a first temperature (T1) and a second temperature (T2) higher than the first temperature, and the difference between the two is calculated to determine the number of crystal defects present in a third layer which results from exclusion of the second layer from the first layer. In this case, the third layer is present between the depth of the first layer and that of the second layer.
It is preferable that the wafer be heated or cooled with a heater or the like in an accurately temperature-controlled state by a temperature controller. By so doing, the wafer can be measured in a uniformly and accurately temperature-controlled state. The wafer temperature may be measured directly with a temperature sensor or may be detected by measuring the temperature of the heater and subsequent calculation based on the result of the measurement. There also may be adopted a method wherein the heater temperature is kept constant, and without measuring the wafer and heater temperatures for each wafer, the intensities of scattered beams at two or more temperatures are measured to determine the depths of defects from the wafer surface and relative dimensional factors of these defects. Further, there may be adopted a method wherein the heater temperature is kept constant, and upon reaching a steady state after putting a wafer in place, the intensity of a scattered beam at any of plural temperatures is measured without measuring the wafer and heater temperatures, thereby calculating the number of defects present in a region from the wafer surface up to an arbitrary depth.
According to the present invention, with only a single laser, an object to be measured is measured at different temperatures, whereby it is possible to measure the difference in intensity between scattered beams caused by the difference in temperature and determine depths and relative dimensional factors of defects.
According to the present invention, measurement is made while changing the temperature of an object to be measured with a single laser, and the difference in intensity of scattered beams caused by the difference in temperature is measured, whereby the number of defects in a specific depth area can be determined.
As to the wafer-heater relation, there may be adopted any of various modes.
For example, a wafer is heated with a single heater and is changed its temperature to two or more temperatures by changing the heater temperature.
Further, a wafer is heated with a single heater and is changed its temperature to two or more temperatures by changing the distance between the heater and the wafer.
Further, a wafer is heated with two heaters in a separate manner and is changed its temperature into two or more temperatures by making the heating temperatures of the two heaters different from each other.
In a further preferred mode for carrying out the present invention, a laser beam having a larger energy than the band gap of silicon is applied obliquely to the surface of a silicon wafer and a scattered beam from defects such as a oxygen precipitates present near the wafer surface is detected. Preferably, if defects present in the depth of about 5 xcexcm from the surface are measured, the wavelength of the laser beam is about 680 nm. It is preferable that, in the depth more than 5 xcexcm, the wavelength of the laser beam is long, while in the depth less than 5 xcexcm, the wavelength of the laser beam is short. If a laser beam having a long wave-length is used, then the measuring area is wide, but the measuring accuracy is poor due to xcex94xcex1 reduction of temperature dependence on absorption coefficient of silicon.
In a still further mode for carrying out the present invention, a laser beam having a larger energy than the band gap of silicon is applied obliquely to the surface of a wafer at a first temperature (T1) and the intensity of a scattered beam from a defect present near the wafer surface is detected, further, the laser beam is directed obliquely to the wafer surface at a second temperature (T2) different from the first temperature (T1) and the intensity of a scattered beam from a defect present near the wafer surface is detected, and on the basis of the difference in intensity between the two scattered beams at the first and second temperatures (T1), (T2) there is calculated the depth of a defect from the wafer surface.
In a wafer defect measuring apparatus in a preferred mode of the present invention, a silicon wafer is placed on a wafer rest, then a laser beam having a larger energy than the band gap of silicon is radiated to the wafer surface obliquely (e.g., at an angle near the Brewster angle), and a scattered beam from defects present in the wafer subsurface is detected. In this apparatus, the wafer rest is provided with a wafer temperature controller to keep the wafer at the same temperature by cooling or heating and the intensity of a scattered beam at any of plural wafer temperatures is measured, or the intensities of scattered beams at two or more wafer temperatures are measured and thereafter relative dimensional factors are obtained.
In another preferred mode according to the present invention, a heater for heating the wafer placed on the wafer rest is suitable for the temperature controller.
In the wafer defect measuring apparatus according to the present invention there may be used a heater or heaters in the following fashion.
(1) A wafer is heated by a single heater and the wafer temperature is changed to two or more temperatures by changing the heater temperature.
(2) While a heater is held at a predetermined temperature, and a wafer is heated by the heater and the wafer temperature is changed to two or more temperatures by changing the distance between the heater and the wafer.
(3) Two heaters are held at different temperatures and a wafer is heated by the two heaters in a separate manner.
Deriving Depth
Various methods are available for deriving the depth. Reference will first be made below to a simple method for deriving the depth referring to FIG. 3.
In the case where a defect present in a certain depth, d, from the surface of a wafer has been detected, the intensity, I, of a scattered beam is determined by the following equation:
I=Aexp(xe2x88x922xcex1d)xe2x80x83xe2x80x83(1)(Lambert-Beer""s law)
where xcex1 stands for an absorption coefficient of Si and A stands for the intensity of the scattered beam in an Si absorption-free condition. I0 is a function of, for example, {circle around (1)} incident beam intensity, {circle around (2)} size and shape of the defect, {circle around (3)} wavelength of incident beam, {circle around (4)} dielectric constant of Si, and {circle around (5)} dielectric constant of defect. Of these factors, {circle around (1)}, {circle around (2)}, and {circle around (3)} are free of changes by temperature, and also as to {circle around (4)} and {circle around (5)}, they undergo such small changes by temperature as can be ignored. Thus, in the equation (1), it is xcex1 that is considered to have a temperature dependence. If the absorption coefficient xcex1 of Si at T1xc2x0 C. and that at T2xc2x0 C. are assumed to be xcex1T1 and xcex1T2, respectively, these absorption coefficients are represented by the following equations:
IT1=Aexp(xe2x88x922xcex1T1d)xe2x80x83xe2x80x83(2)
IT2=Aexp(xe2x88x922xcex1T2d)xe2x80x83xe2x80x83(3)
where IT1 and IT2 stand for the intensities of scattered beams at T1xc2x0 C. and T2xc2x0 C., respectively.
From equations (2) and (3):
d=ln(IT2/IT1)/xe2x88x922(xcex1T2xe2x88x92xcex1T1)=ln(IT2/IT1)/xe2x88x922xcex94xcex1xe2x80x83xe2x80x83(4)
(xcex94xcex1=xcex1T2xe2x88x92xcex1T1)
From equation (2):
A=IT1/exp(xe2x88x922xcex1T1d)xe2x80x83xe2x80x83(5)
Therefore, if xcex1T1 and xcex1T2 are known, or if xcex1T1 and xcex94xcex1 are known, the depth d of a defect is determined from equation (4), and once d is thus known, the intensity of a scattered beam, A, in an Si absorption-free condition can be obtained from equation (5).
Further, equations (2) and (3) can be rewritten in such forms as will be shown below.
As shown in FIG. 3, if a defect is present at a depth of d in a surface layer of Si wafer, then by Snell""s law:
n1sin xcex81=n2 sin xcex82
sin xcex82=n1/n2 sin xcex81
xcex82=sinxe2x88x921(sin xcex81xc3x97n1/n2)
The distance D at which light passes through Si may be written as follows:
D=d/cos xcex82+d=d(1+1/cos xcex82)=d[1+1/cos{sinxe2x88x921(sin xcex81xc3x97n1/n2)}]
Thus,
I=Aexp(xe2x88x92Dxcex1)=Aexp(xe2x88x92xcex1[1+1/cos{sinxe2x88x921(sin xcex81xc3x97n1/n2)}]xe2x80x83xe2x80x83(6)
Deriving a Relative Dimensional Factor
Information on the actual size of a defect can be obtained, for example, by observing one defect which A was measured beforehand with use of a transmission electron microscope (TEM), obtaining Axe2x80x2 of a crystal defect to be observed, and comparing the two.
Also in the calculation of depth, xcex94xcex1 may be used after calculating it on the basis of a measured value obtained with a TEM.
Thus, if a subsurface layer depth and size of a certain defect is once measured with a TEM, then on the basis of the thus-measured value it becomes possible to subsequently measure the depth and to compare the size of the defect in the object to be measured. Even if the dopant concentration of wafer somewhat changes, the measurement can be made with little influence.
The following description is now provided about a method for deriving the depth and a relative dimensional factor of a defect with a higher accuracy.
Given that the intensity of a scattered beam at room temperature (RT) and that at a high temperature (Txc2x0 C.) with respect to a certain subsurface layer defect are IRT and IT, respectively, the following equation is established for this defect in accordance with Lambert-Beer""s law:
In(IT/IRT)=xe2x88x922xc2x7xcex94xcex1xc2x7dxe2x80x83xe2x80x83(7)
where d is the depth of the crystal defect, xcex94xcex1=xcex1Txe2x88x92xcex1RT{xcex1T is an absorption coefficient of Si at Txc2x0 C. (at the incident laser wavelength), and xcex1RT is an absorption coefficient of Si at room temperature}.
{circle around (1)} First, the depth, dTEM, of a certain specific defect from the wafer surface is determined with a TEM or the like. Then, IRT and IT of this defect are obtained and substituted, together with dTEM, into equation (7) to calculate xcex94xcex1.
{circle around (2)} In subsequent measurement, if IRT and IT of another surface layer defect different from the above defect to be measured are measured, it is possible to determine the crystal depth d by using the xcex94xcex1 obtained in {circle around (1)}.
{circle around (3)} As to an estimated value of a relative dimensional factor, it is obtained in the following manner. The value of d obtained in {circle around (2)} is substituted into the following equation (8) or (9):
IRT=Axc2x7exe2x88x922xcex1RTdxe2x80x83xe2x80x83(8)
IT=Axc2x7exe2x88x922xcex1Tdxe2x80x83xe2x80x83(9)
and xcex1RT or xcex1T using a theoretical value or a literature value, as well as IRT or IT of the object to be measured, are given to determine a relative dimensional factor A. The relative dimensional factor A is given as A=Ixc2x7xcex2xc2x7TIxc2x7T0xc2x7xcex4 where I is the intensity of the incident laser beam, xcex2 is the apparatus function, T1 is transmissivity of the incident beam, T0 is transmissivity of scattered light, and xcex4 is the cross section (at the surface), and txe2x80x2 is a reflectance at the surface (after scattering). Although shape, other than dimension, of a defect also contributes to the scattering efficiency, A does not simply depend on only the dimension of a defect, but here A is regarded as a relative dimensional factor in accordance with zero order approximation.
The thus-obtained defect depth d of the object to be measured proved to take a highly accurate value by calculation using xcex94xcex1 which has been obtained on the basis of a value measured with a TEM.
Calculation from the value of d thus obtained shows that the relative dimensional factor A also takes a highly accurate value. It is possible to not only obtain dimensional information of a certain defect but also compare the sizes of two defects.
Although in the above example the depth of a defect is first determined and thereafter a dimensional factor of the defect is obtained, the present invention is not limited to such an example.
Reference will now be made below to an example of obtaining a dimensional factor of a defect directly without obtaining the depth of the defect.
For example, deletion of d from equations (8) and (9) results in:
A=exp{(xcex1TlnIRTxe2x88x92xcex1RTlnIT)/(xcex1Txe2x88x92xcex1RT)}xe2x80x83xe2x80x83(10)
If the intensities of scattered beams are measured at different temperatures with respect to defects of an object to be measured, the relative dimensional factor A can be calculated by substituting the result of the measurement into equation (10) together with xcex1RT and xcex1T. In this case, it is not necessary to measure the defect depth of the object. By measuring the intensities of scattered beams at different temperatures it is possible to directly calculate values of the relative dimensional factor A in two defects. Comparison of the values permits comparison of two crystal defect sizes even without determining the depth of the defect concerned.
It is also possible to calculate the depth of the defect concerned by substituting the relative dimensional factor A thus obtained into equations (8) or (9) together with the intensity of scattered beam and absorption coefficient.
Calculating the Number of Defects
The following description is now provided about how to calculate the number of defects in accordance with the present invention.
In a single wavelength laser scattering tomography, the intensity of a scattered beam, IT, from a certain surface layer crystal defect at a certain temperature Txc2x0 C. can be expressed by the following equation in accordance with Lambert-Beer""s low:
xe2x80x83IT=(Ixcex2TiT0xcex4)xc2x7exe2x88x922xcex1(T)dxe2x80x83xe2x80x83(11)
where d is the depth of the defect, xcex1(T) is the absorption coefficient at the wavelength of the incident laser beam and at the temperature of T, I0 is the intensity of the incident laser beam, xcex2 is the apparatus function, Ti is the transmissivity of the incident beam at the wafer surface, T0 is the transmissivity of the scattered light at the wafer surface, and xcex4 is the cross section (the function of refractive index, size and form of the defect, refractive index Si, and measurement direction).
These are put together as A=Ixcex2TiToxcex4. In effect, the value A reflects the size of a defect. The value A which reflects the size of a defect can be obtained by the following equation using the intensity of a scattered beam, IT, at the temperature Txc2x0 C., absorption coefficient xcex1(T), and depth d:
A=IT/exe2x88x922xcex1(T)dxe2x80x83xe2x80x83(12)
where xcex1(T) is a function of temperature. The higher the temperature, the larger the value of xcex1(T), while the lower the temperature, the smaller the value of xcex1(T). This is proportional to the fact that the higher the temperature, the shallower the penetration of a laser beam, while the lower the temperature, the deeper the penetration of the laser beam.
The present invention is based on the principle that the penetration depth of an incident beam changes with a change of wafer temperature. A detectable depth is controlled by controlling the wafer temperature to an arbitrary value. In this way the number of defects up to an arbitrary depth is determined.
This method does not measure an accurate depth or size of each defect. The depth of a defect can be obtained from equation (11), but in the present invention this may be considered to be one index of a measurable depth.
To make sure, a brief description will be given below of a method for calculating the depth d of each defect from the measurement of scattered beams at two temperatures and in accordance with a laser scattering tomography of a two-temperature type at a single wavelength.
In the case where one defect present at a certain depth d from the surface of a wafer at temperature T is measured, the intensity of a scattered beam, IT, is expressed as follows in accordance with equation (12):
IT=Aexp(xe2x88x922xcex1(T)d)xe2x80x83xe2x80x83(13)(Lambert-Beer""s low)
Therefore, if the measurement is made at two temperatures of T1xc2x0 C. and T2xc2x0 C.:
IT1=Aexp(xe2x88x922xcex1(T1)d)xe2x80x83xe2x80x83(14)
IT2=Aexp(xe2x88x922xcex1(T2)d)xe2x80x83xe2x80x83(15)
where IT1 is the intensity of a scattered beam at T1xc2x0 C. and IT2 is the intensity of a scattered beam at T2xc2x0 C.
From equations (14) and (15):
d=ln(IT2/IT1)/xe2x88x922(xcex1(T2)xe2x88x92xcex1(T1))
If xcex94xcex1=xcex1(T2)xe2x88x92xcex1(T1),
d=ln(IT2/IT1)/xe2x88x922xcex94xcex1xe2x80x83xe2x80x83(16)
Thus, if xcex1(T1) and xcex1(T2) are known, or if xcex1Ti and xcex94xcex1 are known, the depth d of a crystal defect is determined from equation (16), and once d is known, the value A which reflects the size of the crystal defect can be determined from equation (14) or (15).
In this invention, it is easy to measure scattered beams from a wafer because there is no need to move the wafer during the measurement at plural temperatures. The wafer may expand by heating and position of crystal defect may shift. But the shift can be adjusted easily for example, by program.